In an engine of the diesel type air is compressed in a combustion chamber, then fuel is injected under pressure into said chamber. The temperature and pressure conditions in the chamber are such during the injection that there is then combustion of the fuel. This combustion is performed under high pressure and at high temperature and generates the production of polluting gases of the nitrogen oxide type, in particular comprising nitrogen monoxide NO, nitrogen dioxide NO2, etc., more generally known under the name NOx.
In order to limit in particular the polluting emissions of an engine of the diesel type it is known to equip the engine with at least one recirculation circuit. An internal combustion engine is supplied with fresh air and rejects waste gases at the exhaust. For engines of the diesel type it has been found that by mixing some of the waste gases with the fresh air supplying the engine it was possible to influence, or more precisely to reduce, the polluting emissions at the exhaust. A circuit for recirculation of waste gases is commonly referred to as an EGR (exhaust gas recirculation) circuit. Low-pressure or high-pressure EGR circuits exist depending on whether the exhaust gases are drawn at high pressure, that is to say in the proximity of the exit of the gases from the cylinders, or after decompression of the gases further downstream of the cylinders.
In an engine of the diesel type comprising an EGR circuit the air that re-enters the cylinders thus originates in part from the exterior (fresh air) and in part from the EGR circuit (waste gases). A flowmeter is used to measure the flow rate of fresh air admitted into the engine, but the flow rate of recycled gas is not measured. However, it is beneficial to know on the one hand the rate of recirculation gas and on the other hand the content of oxygen of the gaseous mixture entering the cylinders in order to control and manage the engine, in particular so as to order the opening and the closing of the valves of the EGR circuit(s). These parameters (EGR rate and oxygen content) also have an influence on the torque limitation of the engine and the estimation of NOx emissions.
In the known engines, as indicated above, the recycled gas flow rate is not measured by a flowmeter or the like. Information concerning the EGR flow rate and concerning the oxygen content is then obtained on the basis of models that are not very robust and that involve complex calibration and/or with the aid of an oxygen probe, of which the cost price is very high.
Document DE 10 2011 013 481 A1 is known and concerns a method for calculating the temperature in the combustion cycle. The temperature is determined in the phase of valve overlap. The residual mass of waste gases is calculated as well as a reference temperature. It is necessary to know the temperature during the valve overlap and during the opening of the intake valve. This document teaches a method according to the perfect fluid model.
Document DE 10 2009 037 272 A1 is also known and concerns a method for operating an internal combustion engine, said method being based on a perfect gas model.